Sony have this note on their specs for the 10iii with respect to operating frequencies/bands.
"Availability may vary depending on country, carrier, software version and user environment."
Perhaps it would be worthwhile to gather software versions and countries of intended sale etc to see if there’s anything there affecting the ability to get VoLTE working on Sailfish.
One of our local operators also has a requirement for the sim card to be used in a working 5G phone on their network prior to that sim being used in a phone not provided by that operator. This is to get 5G provisioned on the sim. I know we’re talking VoLTE rather than 4G or 5G but wondered if there might be similar requirements around VoLTE for some providers, which might be causing issues.